The most serious exhaust-gas problem of diesel engines is smoke; on the one hand, because of hindering the view on traffic roadways and, on the other hand, because of the characteristic of the smoke to bind other toxic exhaust-gas components such as sulfur oxide and hydrocarbons and to transport the same into the human body. Hot smoke contains significant amounts of soot and occurs because of the incomplete combustion as a consequence of a deficiency of air at high temperatures. The hot smoke occurs at full load or at overload and comprises primarily black soot granules of 0.3 to 0.1 .mu.m.